The present invention relates to an electro-optical distance meter which projects a distance measuring light to an object to be measured, receives a reflected distance measuring light from the object to be measured and measures a distance to the object to be measured based on a light receiving result.
The electro-optical distance meter projects the distance measuring light emitted from a light emitting element to the object to be measured through a distance measuring optical system, receives a distance measuring reflection light from the object to be measured by a photodetector through the distance measuring optical system and measures a distance to the object to be measured based on a time difference between a light emitting timing of the distance measuring light and a light receiving timing of the distance measuring reflection light.
Further, since a drift caused by a temperature change or the like occurs in a light emitting circuit, a light receiving circuit and an arithmetic circuit, the distance measuring optical system has an internal reference optical system in order to suppress an error caused by the circuit.
The internal reference optical system divides a part of the distance measuring light by an optical member such as a half mirror or the like and leads the divided distance measuring light as an internal reference light to the photodetector. As for the internal reference light, too, a time difference between a light emitting timing and a light receiving timing of the internal reference light is obtained, and a measurement distance is calculated based on a difference between a time difference obtained by the distance measuring light and a time difference obtained by the internal reference light. Alternatively, a measurement distance is calculated based on a time difference between a light receiving time of the internal reference light and a light receiving time of the distance measuring light.
By obtaining the difference between the time difference obtained by the distance measuring light and the time difference obtained by the internal reference light, error factors of the light emitting circuit, the light receiving circuit and the arithmetic circuit are offset, and a distance measurement with high accuracy is enabled.
Further, the light receiving timing when the photodetector detects the light is affected by a quality of a received light (brightness, light amount).
Therefore, in order to further improve the distance measurement accuracy, it is desired that qualities of the received lights of the distance measuring light and the internal reference light are also equal.
Conventionally, the internal reference light is obtained by dividing the distance measuring light, and the internal reference light itself has the same quality (a brightness distribution, a luminance speckles or the like are equal) as the quality of the distance measuring light.
However, at a moment when the distance measuring light is divided, the distance measuring light is a luminous flux having a diameter of several mm and thus, the internal reference light is also a similar luminous flux. On the other hand, a light receiving surface of the photodetector has a small diameter of several hundreds μm and thus, a limited part of the luminous flux of the internal reference light is received. A quality of a received light of the internal reference light is affected by a quality of the distance measuring light (the brightness distribution, a luminance speckles or the like), and the quality of the received light changes depending on a position where the received luminous flux is located within the luminous flux of the internal reference light.
Therefore, the quality of the received light of the internal reference light differs from the light receiving quality of the distance measuring tight, as a result, it is possible to cause a distance measurement error.
Further, a support position of the photodetector in the electro-optical distance meter can change in correspondence with a temperature change, and a positional change of the photodetector results in a positional change with respect to the internal reference light and thus, the temperature change can also cause the distance measurement error.